Conventionally, a device has been known as a flow rate measuring device that includes a housing defining a bypass passage therein to take in a portion of an air (main flow) flowing in a duct, and a flow rate sensor disposed in the bypass passage.
For example, Patent Document 1 (JP H8-297039 A) discloses a flow rate measuring device in which an outlet of a bypass passage is open at an outer wall of a housing, and the housing has an oval shape. An outline of the housing disclosed in Patent Document 1 includes a portion (widest portion), which is widest in the outline, at a middle of the housing in a direction of the main flow. The outline is symmetrical about the widest portion.
Since a rate of the main flow is accelerated in an upstream area of the widest portion, a separation of the air is unlikely to occur. Since a rate of the main flow is decreased in a downstream area of the widest portion, the separation of the air may be likely to occur. Hence, when both the upstream area and the downstream area where the likelihood of occurrence of the separation is different have a same shape, it may be difficult to reduce the separation in the downstream area.
Especially, when the outlet of the bypass passage is open at the outer wall of the housing, and when an eddy is generated due to the separation at a downstream area of an airflow near the outlet, the eddy may negatively affect a flow in the bypass passage, and whereby a detection accuracy of the flow rate sensor in the bypass passage may be deteriorated.